The present invention relates to a method of efficiently removing nitrogen oxides (NOx) of relatively low concentration from the gas discharged by ventilation, for example, from various highway tunnels, mountain tunnels, underwater tunnels, underground roads, sheltered roads, etc. (hereinafter referred to collectively as "highway tunnels"), agents for removing NOx by adsorption, and apparatus for purifying NOx-containing gases.
With highway tunnels having a large length and a great vehiclular traffic volume, it is necessary to withdraw the air from the tunnel at a considerable rate for ventilation in order to assure the drivers and passengers of health and increase the least distance of distinct vision. Even with tunnels of relatively short distance, it is practice to ventilate the tunnel to control air pollution due to carbon monoxide (CO), NOx and the like which are concentrically released from the inlet and outlet of the tunnel when the tunnel is located in an urban area or suburban area.
However, if the gas resulting from ventilation is released as it is in the neighborhood, the contaminated gas entails environmental pollution over the area concerned. Especially in cities or suburbs where the pollution due to motor vehicle emissions spreads out two-dimensionally, heavy pollution will then become more widespread. Exactly the same problem as above will be encountered in the case where roadway tunnels or sheltered tunnels are to be constructed as a pollution control measure for existing roads.
The present invention relates to a method of efficiently removing NOx of low concentration from the gas removed from highway tunnels by ventilation, NOx adsorbents for use in this method, and an apparatus for use in practicing the method for purifying the NOx-containing gas.